Skilled or expert

You may choose a location at which to cast explosions of cold. This location may be no more than 10 squares away, must be in sight or (as of 3.6.0) the location of a monster "seen" by telepathy or detect monsters, and must be an open space. The explosion cannot be cast underwater or on the Plane of Water; if used while swallowed, it is automatically cast on your own location.

2 to 9 explosions follow; the first is centered at the location you chose, subsequent ones randomly at any of the nine squares around that point (unless swallowed). Each explosion affects the surrounding squares, as when unskilled/basic.

Any explosions that would be centered on yourself do not take place, but instead do damage as 'zap self' above.

Strategy

The best possible casting scenario for this spell is if you are a level 30 character, 18 intelligence, and are casting at a fire-resistant monster: you can cast up to 9 cold explosions, each of which could hit the monster's square for (6*17 + 3*17) damage, for a total of 1377 damage. This makes this spell potentially the most devastating attack in NetHack, although under normal circumstances magic missile is likely to be better. The cone of cold is most useful for crowd control in Gehennom, where many of the monsters are fire-resistant and thus more vulnerable to the spell. It is also very useful for dealing with vampires transformed into other forms. In Gehennom any vampire bat, wolf or fog cloud killed is, as of 3.6.0, likely to come back to life as a vampire or vampire lord; using a cone of cold might kill the transformed form with an early blast and the risen vampire with one of the subsequent blasts. Coupled with detect monsters, this can be done from another room or corridor, keeping the player safe from the vampiric life-draining attack.

Enhancing to expert skill gives no benefits over skilled for this spell besides a decreased spell failure rate. A level 30 valkyrie with a robe can cast this spell at 2% fail thanks to valkyries' speciality spell bonus. The fail rate is 33% if the valkyrie also wears gauntlets of power (more likely).